1. Field of the Invention
This invention relates to an ophthalmic examination apparatus, and more particularly to an electronic type ophthalmic examination apparatus which uses a laser beam for two-dimensional scanning of the eye fundus, collects the light reflected back from the eye fundus and subjects the light to photoelectric conversion to obtain information about the eye fundus.
2. Description of the Prior Art
Conventionally, in order to examine the eye fundus there are in wide use the method whereby the physician examines the patient's eye directly by means of an ophthalmoscope, and the method whereby a special fundus camera is used to take photographs of the eye fundus. With the advances in recent years of electronic technology, use is also being made of optoelectronic transducers such as imaging tubes and the like in place of the photographic film of the conventional fundus camera, eye fundus information being read out directly in the form of electric signals which are processed and stored in a memory or displayed on a monitor television or the like.
Of these conventional electronic examination apparatuses, one that employs laser scanning and which was developed by a U.S. ophthalmic research organization, the Retina Foundation (see U.S. Pat. No. 4,213,678 and Applied Optics, vol. 19 (1980) page 2991), has attracted attention for the many features it possesses. Specifically, by replacing the light source conventionally used in the CRT-based flying spot scanning type ophthalmic imaging system by a laser beam for eye fundus applications, restricting the incident light beam to a small zone in the center of the pupil and receiving, photoelectrically converting and amplifying the light reflected by the eye fundus from a larger area around the periphery of the pupil, it becomes possible to display on a monitor television a real-time video image of the eye fundus with a low brightness and a high S/N ratio. In addition, it becomes possible to decrease greatly the amount of fluorescent agent that is administered when fluorescent image photography of the eye fundus is to be performed. Also, by modulating the scanning laser beam it becomes possible to examine retina function in the course of observing the eye fundus image, and by utilizing the advantages of the laser beam's depth of focus, the elimination of corneal reflection by polarized light and the monochromatic nature of the light, it becomes possible to provide an excellent diagnostic apparatus.
Various improvements were subsequently made to this new type of ophthalmic apparatus by research groups in a number of countries. Particularly, the Retina Foundation announced a greatly improved ophthalmic examination apparatus (see Japanese Laid-open Patent Application No. 62(1987)-117524 and Applied Optics, vol. 26 (1987) page 1492). Specifically, with this apparatus, in addition to two-dimensional scanning of the eye fundus by the incident laser beam, by also two-dimensionally scanning light reflecting from the fundus and using an optoelectronic detector with an extremely small aperture to detect the reflected light, it becomes possible to provide a marked improvement in the contrast of the eye fundus image thus obtained. In other words, by constituting the entire optical system of reflective-type elements (i.e. mirrors) and scanning both the incident and reflected light beams simultaneously (double-scanning), fixing the scanning of the reflected light acquired by the optoelectronic light detector and detecting only reflected light from a point that is optically conjugate with the fundus of the eye being examined, it became possible to exclude entirely the effect of unrequired light scattered by the optical system of the eye. The apparatus achieved a high level of perfection for clinical ophthalmic examination purposes by enabling retinal blood vessel information to be acquired with sufficient contrast using red light, whereas conventionally green light was required to extract such information, it being considered impossible to obtain an improvement in retinal blood vessel contrast using light with red or longer wavelength; and enabling fundus images of adequate quality for diagnostic purposes to be obtained without the use of an agent to dilate the pupil, i.e. with the pupil in the contracted state to some extent.
The major drawback with this type of apparatus is that the system for controlling the laser beam deflection is difficult. In the reference material cited above (U.S. Pat. No. 4,213,678 and Applied Optics, vol. 19 (1980) page 2991), two mechanical laser beam deflection systems (two sets of oscillating mirrors or galvanometer mirrors) are employed which are operated at a horizontal scanning frequency of 7.8 KHz and a vertical scanning frequency of 60 Hz. But there is the problem that bearings of the horizontal scanning mirror wear severely and its durability is poor because of its high scanning frequency of 7.8 KHz. Moreover, in order to obtain high-definition video images it is necessary to use higher laser beam horizontal scanning frequencies.
For this, in later reference materials (Japanese Laid-open Patent Application No. 62(1987)-117524 and Applied Optics, vol. 26 (1987) page 1492), use is made of a multi-facet mirror for the horizontal optical deflector, driven at a scanning frequency of 15.75 KHz (an oscillating mirror driven at 60 Hz is used for vertical scanning). This scanning frequency is the same as that based on the standard NTSC system raster scan, and is an extremely apt choice in terms of the current state of imaging electronics. It is also very practical with respect to interfacing with peripheral equipment. However, there are many problems in realizing a scanning frequency of 15.75 KHz, as it means a mirror with, for example, 25 faces would have to rotate at 37,800 rpm.
First and foremost are problems relating to service life and durability. Such problems are common to this type of high-mechanically-operated optical deflector including the above-mentioned oscillating mirrors. Even in a high-speed rotationary system using pneumatic bearings, the bearings wear after several thousand starts and stops or metal fatigue leads to a degradation in precision, shortening the system's service life. Secondly, there are the problems of shaft movement, facet inclination and facet division tolerance(facet-to-facet errors). With a multi-facet rotating mirror, wobbling or jitters in shaft play, mirror inclination and mirror division angle errors can produce unevenness of the laser beam raster, and the system is also prone to be influenced by external vibration.
The third point relates to making the system smaller. An apparatus employing a high-speed multi-facet rotating mirror requires large bearings, and because the rotation is limited to a predetermined direction, it is very difficult to reduce the size of the system.
In a mechanical type multi-facet rotating mirror system, the reflective-type deflector is advantageous for simultaneously scanning of the incident beam and the reflected light (double-scanning), and it helps to improve the contrast of the image that is obtained. As stated above, however, there are many problem points relating to the performance of the deflector itself. Also, while in an apparatus that performs double-scanning using reflector-type deflectors there is a major advantage in constituting the entire optical system of reflective elements (mirrors), attempting to introduce an additional refractive optical system can lead to problems. Introducing an additional set of lenses, for example, to change the angle of view can lead to harmful light effects produced by reflection from the lens surfaces. However, such problems can be reduced by choosing an appropriate stop or surface angle. As such, these may be relatively small problems compared with problems arising from the characteristics of the overall apparatus or the deflector. However, in this type of apparatus, in terms of the utility of the apparatus it is extremely important to be able, swiftly and fully, to change the angle of view which establishes the imaging range of the eye fundus that is being examined.
If in the near future there should appear high-definition television, which aims to improve picture resolution and quality, the horizontal scanning frequency will be around 33 KHz. Adapting a mechanical system of deflection to such a high scanning frequency would become even more difficult, owing the problems cited above.
The practical application was then tried of an idea that was announced which involved the use for the horizontal deflector of a non-mechanical acousto-optical device having no moving parts. However, because the acousto-optical device is basically a transmission-type deflector that utilizes diffraction and resolution is limited by the size of the device's aperture and the angle of deflection, it is not conducive to improving the contrast of an image that is obtained using double-scanning of the incident beam and the reflected light. That is, with the optical system of the conventional ophthalmic examination apparatus employing an acousto-optical deflector, unrequired light scattered by parts of the eye-ball other than the fundus could not be excluded, and when the mydriasis was insufficient, there was a problem of deterioration in the contrast of the resulting fundus image.
FIG. 4 shows the optical system of a conventional ophthalmic examination apparatus using an acousto-optical deflector (AOD). See, for example, Japanese Patent Application No. 61(1986)-106688. A laser beam 101 from the laser light source is deflected in one dimension (horizontally) by its passage through the AOD 102. Bracketing the AOD 102 are lenses 103 and 104 to shape the laser beam for impingement on the rectangular aperture of the AOD and to return the beam emerging from the AOD to its original shape. The laser beam scanned by means of the AOD passes through a lens 105, a slit 106 and a lens 107 to a mirror 108. The slit 106 blocks zero-order light and transmits only first-order light. Attached to a galvanometer 109 is a mirror 108 which is oscillated to effect deflection (scanning) in a direction (vertical) that is at right-angles to the direction in which the laser beam is deflected by the AOD. The laser beam scanned two-dimensionally by the mirror 108 passes through a lens 110, is reflected by a mirror 111 and is projected onto the fundus of the eye under examination 113 by an objective lens 112. The light reflected by the fundus goes through the objective lens 112, passes the periphery of the mirror 111 and via a lens 114 is concentrated on the light receiving face of a light-receiving element 115 and undergoes photoelectric conversion. Disposed on the front of the light-receiving element 115 is a filter 116 which filters light of the laser-beam wavelength.
As is apparent from FIG. 4, the light-receiving element 115 receives the major portion of the light emerging from the pupil of the eye being examined 113. As well as the light directly reflected by the fundus, it also detects almost all of the unrequired scattered light from the optical system of the eyeball. Particularly when the pupil is small, this unrequired scattered light causes a deterioration in the contrast of the obtained fundus image. However, with the kind of optical system shown in FIG. 4, because, with respect to the light reflected by the fundus, the light is always deflected two-dimensionally at a point that is optically conjugate with the eye fundus, it has been impossible to detect just the directly reflected light component from the fundus, separately from the unrequired scattered light component.